A magnetic recording medium is widely used as recording tapes, audio tapes, floppy disks and the like. The magnetic recording medium basically comprises a non-magnetic support and magnetic layers formed thereon, the magnetic layer containing a ferromagnetic powder dispersed in a binder.
It is necessary that the magnetic recording media have high levels of characteristics such as electromagnetic characteristics, running durability and running properties. Namely, the audio tapes for music reproduction are required to have higher ability to reproduce original sounds. With respect to the video tapes, excellent electromagnetic characteristics are required such as ability to reproduce original images.
It is known that the electromagnetic characteristics of the magnetic recording media containing the ferromagnetic powders vary considerably depending on the quality of the dispersion of the ferromagnetic powders in the magnetic layers. For this reason, even if ferromagnetic powder having excellent magnetic characteristics is used to improve the electromagnetic characteristics, the excellent magnetic characteristics of the ferromagnetic powder are not reflected in improvements of electromagnetic characteristics when the ferromagnetic powder is poorly dispersed.
As a method for improving the quality of the dispersion of the ferromagnetic powders in the magnetic layers, the method of performing dispersion by kneading for an extended period of time such as for numerous hours has been used in producing magnetic coatings for the preparation of the magnetic layers. However, such dispersion by kneading for prolonged periods of time has the drawback in that the magnetic characteristics of the ferromagnetic powders are adversely affected. In addition, the most efficient operation scheme for the production process is deviated from.
Recently, methods have been proposed in which functional groups are introduced in resin components forming binders so as to give good affinity with the ferromagnetic powders to the binders for the magnetic layers.
For example, such methods for the magnetic recording media are described in JP-A-59-5424 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-59-5423, JP-A-59-40320, JP-A-59-8127, JP-A-61-158023, JP-A-63-275022, JP-A-63-268118, JP-A-61-172213, JP-A-62-107434, JP-A-96727 and JP-A-172710, and such the magnetic coating resins are described in JP-A-60-236814, JP-A-60-238309 and JP-A-61-243933.
However, the object of these methods is to obtain a single magnetic layer.
For example, JP-A-61-172213 proposes to use a copolymer comprising vinyl chloride units, vinyl alcohol units and amine-modified vinyl units as essential components of a binder which is excellent in dispersibility and prevents the ferromagnetic powders from aggregating.
When this copolymer is used for the single layer system, a very hard magnetic layer is obtained and its durability is good. However, the copolymer has the disadvantage of causing imperfect contact with a magnetic head. Further, it has the disadvantage in that the proper adhesion with the non-magnetic support can not be obtained.
Prior ar disclosing the magnetic recording media having the plural-magnetic layer structure includes, for example, JP- A-62-31021. The magnetic recording medium described therein comprises a first magnetic layer formed on a non-magnetic support and a second magnetic layer formed on the first magnetic layer. The first magnetic layer has a lower Young's modulus in the thickness direction than the second magnetic layer. In such prior-art magnetic recording media having the plural-magnetic layer structure, the same binder is used in both of the first and second magnetic layers, as shown in Example 1 of JP-A-62-31021. Specifically, the copolymer of vinyl chloride, vinyl acetate and maleic anhydride is used.
A vinyl chloride copolymer, which is a relatively hard binder, has conventionally been used as a main binder and further in combination with a soft polyurethane resin and the like. It has been found that this copolymer greatly affects the characteristics of the magnetic layer. The phenomenon has been observed that the binder in which carboxyl groups or sulfonic acid groups are introduced improves the dispersibility of the ferromagnetic powder, but hardens the film of the magnetic layer if the functional groups are contained in large amounts. Namely, when the binders each having the same amounts of the functional groups are added to the first and second magnetic layers, respectively, the dispersibility of the ferromagnetic powder in both of the first and second layers is improved, but the film becomes too hard. Consequently, the problem of imperfect contact between the magnetic head and second magnetic layer is particularly encountered in the case of such hardened films.